My Fenix 6 has done this twice, several months apart. The first time I did a factory reset. It happened again today, I deleted files as discussed here: https://forums.garmin.com/outdoor-recreation/outdoor-recreation/f/fenix-5-series/206424/fenix-5x-on-v14-2-won-t-save-activities I still lost my activity but I did not have to factory reset the watch.
